This document specifies a test method for the determination of the brittle crack arrest temperature (CAT). This document is applicable to either ferritic or bainitic, or both steel base metals with a body-centred cubic (BCC) crystal lattice structure that exhibit ductile to brittle transition behaviour. The applicable materials are rolled steel plates. This document is intended for steels with a tensile strength of 950 MPa or less and a thickness greater than 6 mm but not exceeding 200 mm. The range of arrest temperatures is between − 196 °C and + 100 °C. This document also specifies the requirements for test method and test procedures when using the isothermal crack arrest test to judge valid test results under isothermal conditions and in order to determine the crack arrest temperature.